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GSC 2701-2527, a New Multiperiodic High-Amplitude Delta Scuti Variable

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GSC 2701-2527, a New Multiperiodic High-Amplitude Delta Scuti Variable Bull. Astr. Soc. India (2013) 41, 173–182 GSC 2701-2527, a new multiperiodic high-amplitude Delta Scuti variable D. P. Kjurkchieva,1∗ D. P. Dimitrov,2y S. I. Ibryamov,2z and M. Srinivasa Rao3x 1Department of Astronomy, University of Shumen, 115 Universitetska Str., 9712, Shumen, Bulgaria 2Institute of Astronomy and Rozhen Observatory, BAS, 72 Tsarigradsko Chaussee Blvd., 1784, Sofia, Bulgaria 3Indian Institute of Astrophysics, Bangalore 560034, India Received 2012 October 16; accepted 2013 March 02 Abstract. Our BVI photometric observations reveal that GSC 2701-2527 is a high- amplitude δ Scuti star with quite low temperature. The frequency analysis exhibits that it is multiperiodic pulsator. The obtained period ratio f1/ f2=0.791 is higher than the canonical value. The large value of the period ratio f1/ f3=0.813 implies nonradial nature of the third mode. Using empirical relations for the δ Scuti stars we determined the global parameters of GSC 2701-2527: luminosity 26.2 L , radius 3.95 R , mass 2.18 M , and distance 506 pc. Keywords : methods: observational – methods: data analysis – stars: variables: δ Scuti – stars: fundamental parameters – stars: individual GSC 2701-2527 1. Introduction The δ Scuti stars are short-period pulsating variables located at the intersection of the classical Cepheid instability strip with the MS with periods between 0.02 d and 0.25 d, masses from 1.0 M to 3.0 M , radii from 1.2 R to 6.1 R (McNamara 2000) and spectral types from A to F. The δ Scuti stars are dwarfs or subgiants that have not yet reached the red giant branch with log g between 3.4 and 4.4 (McNamara 2000; Alcock et al. 2000). ∗email: [email protected] yemail: [email protected] zemail: [email protected] xemail: [email protected] 174 D. Kjurkchieva et al. Nowadays it is assumed that the δ Scuti stars pulsate with a large number of radial and nonra- dial modes and represent a transition between the Cepheid-like large-amplitude radial pulsation of the instability strip and the nonradial pulsation occurring in the hot half of the HR diagram (Dawson et al. 1995; Breger et al. 1999; Breger 2000). Due to their low-order acoustic and gravity modes (driven by the opacity mechanism acting mainly in the HeII ionization zone) the δ Scuti stars are considered as excellent probes for study of the stellar interiors. The observations before several decades revealed that the low amplitude δ Scuti stars (LADS) show a large variety of nonradial modes and complex light variability (multiperiodicity, close doublets of frequencies, amplitude variations, etc.) while the high amplitude δ Scuti stars (HADS) were thought as classical radial pulsators, mostly monoperiodic (double mode in some cases, but always radial modes). But recent observational studies show that many HADS stars are also multiperiodic variables or undergo simultaneously radial and nonradial pulsations (Walraven et al. 1992; Garrido & Rodrýguez 1996; Arentoft et al. 2001; Zhou 2002; Poretti 2003; Poretti et al. 2005). The only remarkable difference is that the nonradial modes in HADS stars have a much smaller amplitude (order of magnitudes) than the radial modes and that period and amplitude variations can be considered as small perturbations of a mode always clearly visible in the light curve. Therefore, nowadays the phenomenology of the HADS stars becomes similar to that of the LADS stars and their role for the asteroseismology increases. Moreover, since HADS stars obey a period-luminosity (P-L) relation, they are used as standard candles to find distances (McNamara et al. 2007). Therefore, any new detection of a HADS star can be a valuable contribution to the asteroseismology and astrophysics. Due to the short periods, detectable photometric amplitude (ranging from 0.01 mag to sev- eral tenths) as well as the high luminosity (absolute magnitudes ranging from 0.4 to 2.5), the oscillations of the δ Scuti stars can be detected from the ground. A list of pulsating δ Scuti stars can be found in the catalogs of Rodriguez et al. (2004) and Garcia et al. (1995) while lists of pulsating δ Scuti stars in stellar systems are given by Lampens & Boffin (2000) and Soydugan et al. (2011). Recently the discovery of new δ Scuti stars has been accelerating due to large observing programs such as HIPPARCOS, MACHO, ASAS, OGLE, etc. Applying our code (Dimitrov 2009) for stellar variability detection, several hundred δ Sct candidates were identified from surveying of the NSVS data base. One of them was the object NSVS 8611544 = NSVS 8666435 (also identified as GSC 2701-2527, SAO 70932, BD+30 4323, TYC 2701-2527-1, 2MASS J21070986+3052346, SWASP J210709.87+305234.6). Further we will use the designation GSC 2701-2527. This paper presents our observations of GSC 2701-2527 and the analysis that allowed us to conclude that it is a HADS variable and to estimate its modes and some global parameters. GSC 2701-2527, a new multiperiodic high-amplitude Delta Scuti variable 175 2. Observations and data reduction 2.1 The previous observations The target GSC 2701-2527 has been used as a field (nonvariable) star for determination of the color excess of the Cepheid DT Cyg by Feltz & McNamara (1976) who determined V=9.74 mag, b-y=0.329, E(b-y)=0.093, distance 320 pc. The star is noted as variable by Bernhard & Srdoc (2011). We found photometric unfiltered (close to R band) data of the target in the NSVS database (Wozniak et al., 2004) as well as SuperWASP database (Pollacco et al. 2006; Butters et al. 2010). The periodogram analysis of these data allowed us to derive the ephemeris HJD(Max) = 2454274:7665247(10) + 0:149427(5) ∗ E: (1) Figure 1. The folded light curves of the NSVS and SWASP data We phased the NSVS and SWASP data with the ephemeris (1). The shape of the folded light curves (Fig. 1) as well as the derived period implied a δ Scuti star classification. 2.2 Rozhen observations and their preliminary analysis Our CCD photometric observations of GSC 2701-2527 in BVI bands were carried out on 2010 August 25 at the Rozhen National Astronomical Observatory with the 60-cm Cassegrain tele- scope using the FLI PL09000 CCD camera (3056 × 3056 pixels, 12 µm/pixel, field of view 27.0 x 27.0 arcmin with focal reducer). The exposures were 40 s in B, 30 s in V, and 20 s in I band. The average photometric precision per data point was 0.01 mag. The standard IDL procedures (adapted from DAOPHOT) were used for reduction of the 176 D. Kjurkchieva et al. Table 1. Coordinates, magnitudes and colours of the target and standard stars Star GSC ID RA (2000) DEC (2000) V B-V V-I Sp type Var 2701-2527 21 07 09.87 30 52 34.6 9.64 0.39 0.47 F2 C1 2701-2385 21 07 42.96 30 46 55.9 9.88 0.27 0.30 C2 2701-2191 21 06 47.78 30 49 39.2 10.79 0.30 0.34 C3 2701-1362 21 07 30.19 31 02 42.6 9.27 -0.06 -0.10 A0 C4 2701-2362 21 07 12.51 30 49 16.3 11.32 1.08 1.17 C5 2701-2063 21 07 30.27 30 57 54.6 11.75 0.60 0.71 photometric data. For transition from instrumental system to standard photometric system we used standard stars of Landolt (1992) and standard fields of Stetson (2000). The standard stars in the observed field (Fig. 2) were chosen by the criterion to be constant within 0.01 mag during the observations in all filters. Table 1 presents their magnitudes and colors V, B-V, and V-I from the NOMAD (Zacharias et al. 2004) and USNO-B1 catalogs while the spectral types are from the SIMBAD database. Figure 2. The field of GSC 2701-2527 Rozhen observations of GSC 2701-2527 cover more than two light cycles and Fig. 3 presents the folded light curve phased according to the ephemeris (1). A low-dispersion spectrum of the target (Fig. 4) was obtained on 2011 May 21 by using the focal reducer FoReRo2 attached to the 2-m telescope at Rozhen. CCD camera VersArray 512, grism with 300 lines/mm and slit width of 0.11 mm were used. The spectrum covers the range 5500-7100 A. Its reduction was performed using IRAF packages by bias subtraction, flat fielding, cosmic ray removal, one-dimensional spectrum extraction and wavelength calibration. Emission lines of a Rb source were used for wavelength calibration. We classified the target on the basis of several considerations. GSC 2701-2527, a new multiperiodic high-amplitude Delta Scuti variable 177 Figure 3. The Rozhen folded light curves of GSC 2701-2527 Figure 4. Low-dispersion spectrum of GSC 2701-2527 (a) The folded light curves reveal round minimum, relative sharp maximum (Figs. 4 and 5) and asymmetry that are typical for a δ Scuti star. Moreover, all attempts to model the GSC 2701-2527 light curves by eclipses of close binary configuration turned out unsuccessful. (b) The derived period 0.149427 d is appropriate for a δ Scuti star. (c) The lack of a exact repeating of the variability even during the two consecutive light cycles (Fig. 5) is typical for a δ Scuti star. (d) GSC 2701-2527 becomes redder at the light minima and bluer at the light maxima (Fig. 5) similarly to the δ Scuti stars. (e) The mean light variability amplitudes of GSC 2701-2527 in B, V and I filters are corre- spondingly 0.33±0.01 mag (range 10.25-9.92 mag), 0.25±0.01 mag (range 9.74-9.49 mag) and 0.15±0.01 mag (range 9.11-8.96 mag).
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